The present invention relates to an antenna applied to a receiving apparatus operable to receive signals transmitted from an artificial satellite.
Recently, digital radio receivers operable to receive satellite waves or ground waves have been developed. A user can listen to radio broadcasts with these digital radio receivers. These digital radio receivers have been put into practical use in the United States. These digital radio receivers are mounted on mobile stations such as automobiles and operable to receive electric waves, the frequencies of which are in the frequency band of about 2.3 GHz. That is, the digital radio receiver is capable of listening to mobile radio broadcasts. Since the frequency of the electric waves to be received is about 2.3 GHz, the receiving wave-length (the resonance wave-length) λ is about 128.3 mm. In a case where these digital radio receivers receive ground waves, the ground waves are transmitted in such a manner that satellite waves are first received by a global station and then transmitted again by linear polarization after the frequency has been slightly shifted.
This type of receiving system for receiving satellite waves or ground waves includes an antenna device and a radio tuner. The antenna device receives satellite waves or ground waves and then outputs a receiving signal of high frequency. The radio tuner demodulates the receiving signal and outputs a voice signal.
In the related-art antenna device, for example, it is possible to use a plane antenna disclosed in Japanese Patent Publication No. 2001-24428. The plane antenna disclosed in Japanese Patent Publication No. 2001-24428 includes: an upper case; a plane antenna element arranged on a reverse side of the upper case while a predetermined interval is being formed between the plane antenna element and the reverse side of the upper case; a ground sheet arranged while a predetermined interval is formed between the ground sheet and the plane antenna element; a feeding conductor arranged between the plane antenna element and the ground sheet; a circuit board attached to a lower face of the ground sheet; and electronic parts mounted on the circuit board.
In the related-art antenna device, it is necessary to provide a ground sheet used for antenna emission which is arranged separately from the circuit board. On the other hand, the ground sheet is attached to one side of the circuit board and electronic parts are mounted on the other side of the circuit board.
The circuit board is a printed board on which a control circuit and an information processing circuit used for communication are mounted. On the circuit board, a CPU for controlling the overall antenna device and a high frequency oscillator for supplying a high frequency signal to an antenna wire or an antenna face are mounted. That is, on the same board, the digital circuit and the analog circuit exists while being mixed with each other.
From such a circuit board, electromagnetic noise is emitted. The emitted electromagnetic noise spreads in a space. On the other hand, an insulating material is inserted into a gap formed between the circuit board and the ground sheet used for antenna emission. However, it is inevitable that an electric potential difference is generated between a ground electric potential of the circuit board and a ground electric potential of the ground sheet for antenna emission. Further, it is inevitable that a fluctuation is caused in the electric potential difference. Due to this fluctuation of the electric potential difference, electromagnetic noise generated from the circuit board enters the antenna element and a performance of the antenna device is deteriorated.
Therefore, in the related-art antenna device, it is necessary to interpose an electromagnetic shielding sheet and an insulator sheet between the ground sheet for antenna emission and the circuit board. Therefore, it becomes difficult to reduce the size of the antenna device. Further, it becomes difficult to reduce the thickness of the antenna device.